This invention is directed to a method for treating an aqueous slurry of blast furnace flue dust wherein carbon and iron bearing particles are separated from contaminating particles, i.e. zinc and lead bearing particles, and are recovered and reused in the blast furnace.
Optionally, the water in the slurry can be treated to precipitate dissolved zinc and lead values which can be separated from the water to thereby upgrade the water to acceptable environmental standards.
Basic iron is produced by smelting iron oxides in the presence of carbon, fluxes and pressurized air in vertical shaft or blast furnaces. As the air passes upwardly in the furnace, reactions occur in which iron oxides are reduced to molten metallic iron which is collected in the hearth of the furnace. Large volumes of waste gases and minute solid particles are produced in the process. The solid particles are carried out of the top of the furnace in the waste gases. The waste gases are initially dry treated to remove relatively coarse particles and are then wet electrostatically treated or wet scrubbed with fine water sprays to remove a substantial portion of the relatively fine particles so that the waste gases meet acceptable environmental standards when discharged into the atmosphere.
The relatively fine particles removed from the waste gases are referred to as blast furnace flue dust and are collected as an aqueous slurry in particle settling devices, for example thickeners. Generally, the size of the fine particles in flue dust is less than about 100 mesh. A large weight percentage of the particles are carbon and iron bearing particles which, if recovered, could be recycled to the blast furnace.
The flue dust also contains contaminants, particularly zinc and lead bearing particles. Minute quantities of zinc and lead can be tolerated in the blast furnace. However, large quantities of such elements cannot be tolerated because both zinc and lead are known to attack the refractory lining the interior surface of the metallic shell of the blast furnace. Zinc and lead cause the refractory to spall thereby decreasing the thickness of the refractory and reducing the life of the lining and shortening its life. Zinc and lead can also cause scaffolding which impedes flow of materials in the furnace. It is, therefore, essential to separate the zinc and lead bearing particles from the carbon particles and iron bearing particles prior to reuse of the flue dust as sinter or pellets which constitute part of the burden charged into the blast furnace.
Several prior art processes have been suggested for separating zinc and lead bearing particles from blast furnace flue dust. In some processes, addition agents which combine with zinc and lead are added to the flue dust and the zinc and lead removed as complex compounds. Other processes suggest heating the flue dust to vaporize zinc and lead which are then condensed on cold surfaces. Still other processes involve balling the flue dust and vaporizing the zinc and lead during the pelletizing or heat-hardening step. None of the above processes have proven to be commercially feasible. Still other processes in which the flue dust is comminuted and the comminuted particles are subjected to wet classifying means to separate zinc and lead bearing particles from the flue dust are known. Unfortunately, in such processes, a quantity of slimes is formed. The slimes adversely affect the separation of zinc and lead bearing particles. In which case, it is necessary to separate the slimes from the flue dust prior to separating other particles from the flue dust.
Blast furnace flue dust is collected as an aqueous slurry, therefore, some of the zinc and lead values may dissolve in the water particularly if the water is acidic or highly basic, thus contaminating the water. Such water usually cannot be wasted in environmental waters. Recycling of the acid water in the plant results in a build-up of dissolved zinc and lead values which can poison the system. It is essential that a substantial portion of the dissolved zinc and lead values be removed from the water to upgrade its quality prior to disposal or reuse.
There is, therefore, a need for a simple, efficient method for separating carbon and iron bearing particles from other particles such as zinc and lead bearing particles in an aqueous slurry of blast furnace flue dust whereby a substantial portion of the carbon and iron bearing particles contained therein can be recovered and reused as part of the solid burden charged into the blast furnaces.
It is an object of this invention to provide a simple, efficient method for separating a substantial portion of carbon and iron bearing particles from particles of zinc and lead bearing particles contained in blast furnace flue dust, recovering the carbon and iron bearing particles and preparing them for reuse in a blast furnace.
It is another object of this invention to provide a simple efficient treatment of an aqueous slurry of blast furnace flue dust whereby a substantial portion of the carbon and iron bearing particles are separated from particles of zinc and lead bearing particles contained therein and, optionally, dissolved zinc and lead values are precipitated and removed from the water to upgrade the water to acceptable environmental standards.